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Here’s my new Season Extension slideshow that I presented recently for the Allegheny Mountain Institute Farm at Augusta Health and the Center for Rural Culture in Goochland. Click the diagonal arrow icon to see it full screen.

Since Hurricane Michael passed by, temperatures have plummeted. I dusted off our Frost Alert List. First is the “Grab and Run” list of what to do. Then follows a list of factors to consider to help you forecast whether or not you are likely to get a frost. We take the night-time low temperature for our nearest town (7 miles away) and subtract 5F to predict what temperature we’ll get.

Frost Alert List

Task

Crop

Notes

Harvest all edible

Asparagus beans

Harvest all edible

Eggplant

Harvest all edible

Okra

Harvest all edible

Tomatoes

Incl green

Harvest all edible

Peppers exposed to the sky

Harvest all edible

West Indian gherkins

Harvest all edible

Pickling cucumbers

Harvest all edible

Corn

Harvest all edible

Green bean plantings past their prime

Thick row cover

Late Beans #5,6

Uncover once mild again

Thick row cover

Summer squash and zucchini

Spring hoops or none. Ditto

Thick row cover

Slicing cucumbers

Spring hoops or none. Ditto

Thick row cover

Celery

Double hoops -leave covered

Thick row cover

Last lettuce bed

Double hoops – leave covered

Set sprinklers

Slicer tomatoes

Overnight from before 32F till after sun shines on plants

Set sprinklers

Roma paste tomatoes and peppers

Ditto

Set sprinklers

Other vulnerable crops

Ditto

Frost is more likely on our farm if. . .

Date is after 10/14

Daytime high temperature is less than 70F (21C)

Sky is clear

Sunset temperature is less than 50F (10C)

Dewpoint forecast (Louisa minus 5) is less than 43F (6C).

Wunderground 3.30pm forecast for Louisa low temp is less than 38F (3.5C)

Little or no breeze (But see last point in list)

Soil is cool and dry

If temps are falling fast, the sky is clear, and it’s windy (esp from NW), it may be polar air moving in and we could get a hard freeze.

If you are pondering hoop systems for rowcover, here are our winter double hoops. The inner hoop is from 9 or 10 gauge wire, bent round a jig to make eyes. The outer hoop is 22 gauge wire and has the ends bent into hooks. We set the inner hoops every 6′ (2 m) along the bed, fit the rowcover, and roll its edges around wood stakes. then we add the outer hoops, hooking them into the ground-level eyes of the inner hoops. Lastly we tension the rowcover lengthwise. The outer hoops stop the rowcover from blowing away, and hole it in place when we push the edges up to harvest.

Our average first frost date is October 14. Actually from our own records it has averaged 10/22 over the last 11 years. So any time now, we could get a frost. We had a very light patchy frost 10/10 with a recorded overnight low of 36F, but now we are having a warm spell (very nice). It’s good to be prepared. We harvested our sweet potatoes (not a good yield, too many deer). We are getting ready to harvest our white potatoes.

DIY weather-forecasting

I recommend learning the local weather patterns by keeping records and watching what happens. Here’s what I’ve learned about ours:

Our mid-Atlantic climate is controlled by three weather systems,

mainly by moisture from the Gulf of Mexico,

the Bermuda High Pressure area in summer,

recurrent waves of cold Canadian air in winter.

Rain (fairly evenly distributed throughout the year in our county)

has slight peaks in January, February and March

and again in early June and August.

Some parts of our area can experience long periods of drought.

September-November is the drier season but it’s also the hurricane season, so the net result is very variable.

We use Wunderground.combut subtract 5F° from their forecast night lows for our nearest town, and mentally downgrade the chance of rain by 10%, as rain often passes us by as it scoots along the river valley north of us.

Local Information

According to a Freeze / Frost Occurrence Data pdf for Virginia which I have, in Louisa County the threshold of 36F has a 50% likelihood on Oct 3; the 32 F threshold has a 50% likelihood by Oct 13 and the 28F threshold is as likely as unlikely by Oct 27. The 90% chances occur by Oct 14, Oct 28 and Nov 13 respectively. The data for spring are available here. probably the site has the fall info too, but I’m out of time to look, due to three hours without internet this afternoon.

Frosty daikon leaves.Photo Bridget Aleshire

Help predicting frost

We keep this reminder list handy at this time of year, when trying to make our own frost forecasts:

Frost is more likely at Twin Oaks if:

The date is after 10/14 or before 4/30.

The daytime high temperature was less than 70°F (21°C).

The sky is clear.

The temperature at sunset is less than 50°F (10°C).

The dew point forecast is low, close to freezing. Frost is unlikely if the dew point is 43°F or more.

The Wunderground 3.30pm forecast low for Louisa Northside is 37°F (3°C) or less.

The soil is dry and cool.

The moon is full or new.

There is little or no breeze, although if temperatures are falling fast, the wind is from NW and the sky is clear, then polar air may be moving in, and we’ll get a hard freeze.

We also make for ourselves a Frost Alert Card, so we can quickly know which beds we need to attend to if we decide a frost is likely. (RBs means raised beds)

is a 126 page book which includes explanations of freezes and frosts (both radiation frosts and advection frosts), and a chapter on recommended methods of frost protection for food crops. probably none of us will need the information on wind machines and helicopters (maybe little drones work for small scale growers?). The publication includes frost forecasting and frost damage physiology (for growers not in a panic about immediate frosts, who want to understand what the plants are going through).